Broom psyllids and Liberibacter

In 2011 the bacterium Candidatus Liberibacter europaeus (hereafter Leu) was reported in NZ Scotch broom (Cytisus scoparius). A retrospective risk assessment found strong evidence to suggest Leu got into NZ as a passenger on the broom psyllid, Arytainilla spartiophila, which was introduced in 1993 as a biocontrol agent.

The evidence is as follows: firstly, methods used to import broom psyllids from the UK provided a likely pathway for inadvertent entry of Leu into NZ, as Ca. Liberibacter was unknown to science at the time. Secondly, Leu in NZ Scotch broom plants was significantly correlated with the presence of broom psyllids. Leu was absent from plants at sites without psyllids.

An alternative hypothesis, that Leu was present in NZ pre-1993, and maintained in cultivated plants via cuttings/grafting, or in wild Scotch broom populations via shared parasitic plants, was determined improbable: Leu was not found in ornamental brooms or pears (Pyrus communis L.) and parasitic plants were too uncommon to maintain Leu in wild Scotch broom. Finally, Leu in Scotch broom and psyllids in the UK shares close genetic resemblance with Leu that was found in NZ, further strengthening the link between the introduction of the psyllid and the arrival of Leu.

It remains uncertain whether Leu is pathogenic in Scotch broom or symptomless, as it is in pears. Unpredictable impacts of Ca. Liberibacter spp. in different plant species means that psyllids considered as future weed biocontrol agents will need to be free of these potential plant pathogens before being released into NZ. This accidental introduction of Leu into NZ via a weed biocontrol agent happened because detection methods for unculturable organisms were not developed at the time. As molecular methods continue to improve, molecular characterisation of insect microbiomes such as those used in our analysis of Leu in broom are likely to become an essential procedure for pre-release safety screening of biocontrol agents that are potential vectors of plant pathogens.

Broom psyllids and Liberibacter

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